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Linné on line arrow Physics and the Cosmos arrow Physics and the Cosmos arrow Crystals – nature putting itself in order arrow Liquid Crystals

Liquid Crystals

A liquid crystal is something in between a liquid and a crystal. In a liquid the molecules are not ordered. This means that they do not have fixed positions nor any fixed orientation or direction. In a liquid crystal the molecules still do not have fixed positions but their orientation is well determined such that they all point in a certain direction. This property can be exploited to make different kinds of displays (called LCD for short). The simplest example is the type of display used in a digital watch or a pocket calculator. In this case one uses liquid crystals with molecules that are optically active, which means that they can change the polarization of light.

Usually one uses a so called TN-cell which can rotate the polarization of the light a quarter of a turn given that the length of the cell is long enough compared to the wavelength of the light. By placing a vertical polarization filter before the cell and a horizontal one after the cell we get a system which lets through light.

This property can be changed by applying an electric field across the TN-cell which makes the molecules change direction so that they no longer change the polarization of the light. This way no light comes through the system since the two polarization filters are perpendicular to each other.

In a simple display this property is used together with the polarization filters. By placing a mirror behind the second filter one gets a "light-cell" which either reflects light and thereby looks bright or absorbs the light and becomes black. With the help of transparent electrodes one can apply different electric fields to different parts of the display and thereby create a pattern.